The four prong outlets with four wires are electrically safer. As Rcik points out though, you have to have four wires present if you want to upgrade to the 4 prong outlet (or be willing to run a new wire).

The reasons that the four prong outlets are safer is the same reason you need a four wire feeder to electrical panels downstream of the service equipment. The only interconnection between the ground and neutral you want in your electrical system is at the service equipment.